In modern gas turbine engines, the blades and vanes in the high pressure turbine section are exposed to temperatures in excess of 1000 degrees C. for extended periods of time. Oxidation distress of turbine components is commonly inhibited by the application of platinum aluminide coatings. Protection provided by Pt-aluminide coatings is due to selective oxidation of aluminum to form an alumina (Al.sub.2 O.sub.3) scale that grows very slowly at high temperature by a diffusion process. Impurities within the coating, notably sulfur, can segregate to the interface between the coating and the alumina scale, weaken the interface, and thus promote spalling of the protective oxide scale. Periodic oxide spalling accelerates the consumption of aluminum from the Pt-aluminide coating and reduces the oxidation life of the component. Impurity induced oxide spalling of the protective oxide also limits the life of thermal barrier coatings that utilize Pt-aluminide coatings as a bond coating.
It has been observed that commercial electroplating processes that are used to apply a thin (1 to 5 micron) layer of platinum to the component (prior to diffusion and aluminizing) contribute significantly to the amount of undesirable impurities present in a Pt-aluminide coating. Consequently, there is a need for a plating process that greatly reduces the concentration of impurities (specifically, S, Cl, and P) present within the plating to levels that are comparable or preferably below the levels present within the superalloy substrate.
In the production of platinum modified aluminide diffusion coated gas turbine engine components, such as blades and vanes, the components are conveniently electroplated to deposit platinum metal on their gas path surfaces prior to aluminizing. Some prior art plating baths employ hexachloroplatinic acid (H.sub.2 PtCl.sub.6) as a source of platinum. Examples include the phosphate buffer solution as described in U.S. Pat. Nos. 3,677,789 and 3,819,338 or an acid chloride bath similar to that outlined by Atkinson in Trans. Inst. Metal Finish. vol. 36 (1958 and 1959) page 7. Sulfate solutions also have been used in the past which utilize a P salt [(NH.sub.3).sub.2 Pt(NO.sub.2).sub.2] ] precursor as described by Cramer et al. in Plating vol. 56 (1969) page 516 or H.sub.2 Pt(NO.sub.2).sub.2 SO.sub.4 precursor as described by Hopkins et al. in Plat. Met. Rev. vol. 4 (1960) page 56. Finally, some platinum aluminide coating procedures utilize a platinum Q salt [(NH.sub.3).sub.4 Pt(HPO.sub.4)] bath as discussed by Albon, Davis, Skinner and Warren in U.S. Pat. No. 5,102,509. Conventionally well known platinum plating baths contain high concentrations of sulfur and/or phosphorous and/or chlorine, and the deposition reactions in all these baths involve complex ions with ligands containing sulfur and/or phosphorous and/or chlorine.
A disadvantage in using these electrolytes is that it results in the co-deposition of P, S and Cl impurities into the Pt coating. The presence of these impurities essentially reduces the life of the Pt-aluminide coating, the thermal barrier coating that uses a Pt-aluminide coating as the bond-coat, and consequently the life of the article.
One proposed solution to this problem is disclosed in Warnes et al. U.S. Pat. No. 5,788,823. Warnes teaches a method of improving the oxidation resistance of platinum modified aluminide diffusion coatings by electroplating platinum onto a substrate using an aqueous caustic solution based on an platinum hydroxide and alkali hydroxide or alkaline earth hydroxide. Use of the hydroxide plating solution significantly reduces the presence of such harmful impurities as phosphorus and/or sulfur and/or chlorine in the platinum deposit and thus in the platinum modified aluminide diffusion coating formed on the substrate.
One disadvantage to using the electrolyte disclosed in Warnes is that after being made up, it rapidly becomes turbid and a deposit of hydrated Pt oxide forms on the anode. With more prolonged use a heavy flocculent precipitate of various Pt compounds is formed, whereby the effective Pt content of the bath is reduced and Pt coatings become dull, patchy and loosely adherent. To avoid this problem, the addition of Sodium Chloride (NaCl) and Sulfuric Acid is suggested which leads to increased cost and a contamination of the electrolyte and Pt coatings with Cl and S impurities.
Another disadvantage to this electrolyte is that it absorbs carbon dioxide from the atmosphere. As a result, periodic addition of NaOH (KOH) is necessary which leads to build-up of alkali carbonates M.sub.2 CO.sub.3 in the electrolyte, incorporation of the alkali metal into the Pt coating and consequently a reduction of the electrolyte lifetime.
Accordingly, there exists a need for an electrolyte that can be used on electrolytic plating of platinum that results in reduced Cl, S, or P contaminant production and platinum coatings that are not dull, patchy and loosely adherent.